A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”—direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In a device manufacturing method using projection lithographic apparatus, the minimum feature size, often referred to as critical dimension (CD), is determined by the wavelength (λ) of the exposure radiation and the numeric aperture (NA) of the projection system. Various techniques have been developed to reduce CD and these are commonly combined into a factor known as k1 so that CD=k1·λ/NA. With current technology, it may not be possible to image a feature smaller than determined by a k1 factor of about 0.25 in a single exposure-develop-process cycle. However a smaller feature may be imaged with a double exposure and double process technique.
In a double exposure technique, a single layer of resist is exposed twice—either with two different patterns or with the same pattern but with a positional offset—before being developed. In a double process technique, a first layer of resist is exposed and developed and then the substrate is etched, transferring the pattern to the substrate, then a second layer of resist is applied to the substrate. The second layer is then exposed, developed and the substrate etched so that the final pattern in the substrate is produced by the combination of the two etch steps. A double process technique may be used to create a wide variety of useful structures but may be slow, taking one or two days because of the etch steps and the need to remove the substrate from the lithocell—comprising a lithographic apparatus and process apparatus such as a spin coater, developer and bake & chill plates—to perform the etch steps. Although a double exposure technique may be performed much more quickly, the range of structures it may be used for is more limited.